Process for producing carboxylic acids and esters

ABSTRACT

A method and apparatus for controlling the quantity of gas in a stream of liquid by circulating the stream of liquid within a vessel and introducing the gas into the vessel into contact with the stream of liquid both above and below the surface of the stream of liquid. The quantity of the gas present in the circulating stream of liquid is controlled by selectively adjusting either the velocity of the stream of liquid within the vessel or the depth of the introduction of the gas into the vessel below the surface of the stream of liquid.

United States Patent Wesselingh [4 1 Sept. 12, 1972 [54] PROCESS FORPRODUCING CARBOXYLIC ACIDS AND ESTERS Johannes A. Wesselingh, Amsterdam,Netherlands Assignee: Shell Oil Company, New York, NY.

Filed: May 28,1970

Appl. No.: 41,462

Inventor:

Foreign Application Priority Data July 10, 1969 Netherlands ..6910591US. Cl. ..260/497 R, 260/4109 R, 260/413, 260/531 R, 260/533 A, 260/540,261/87, 261/93 [51] Int. Cl ..C07c 51/14 [58] Field of Search....26ll87,93; 260/533 A, 497 R References Cited UNITED STATES PATENTS Shirley..261/93 2,865,618 12/1958 Abell ..261/93 2,928,665 3/1960 Epprecht..261/87 2,293,183 8/1942 Walker ..261/93 3,527,779 9/1970 Paulis et al..260/533 A Primary Examiner-Lorraine A. Weinberger AssistantExaminer-Richard D. Kelly Attorney-Harold L. Denkler and Theodore E.Bieber [5 7] ABSTRACT 8 Claims, 1 Drawing Figure PROCESS FOR PRODUCINGCARBOXYLIC ACIDS AND ESTERS BACKGROUND OF THE INVENTION liquid, in whichthe gas is dispersed in the form of bubl bles, through supply of gasthat is present over the liquid. The invention also relates to anapparatus that is suitable for the process according to the invention tobe carried out therein.

2. Description of the Prior Art In carrying out reactions in the liquidphase in reactors, where gas present in the reactor has to be taken upin a liquid likewise being present in the reactor (gas present in theliquid being consumed as a result of the reaction), in many cases it ispossible for the quantity of gas in the liquid to be kept at the desiredlevel (even when this level is high) by imparting a vigorous motion tothe liquid and to the interface between liquid and gas. Sufficientagitation in a reaction vessel of a not too complicated shape cangenerally be effected by vigorous stirring. With such reaction vessels,problems may occur in relation to the control of the temperature,because with highly exothermic reactions it is mostly impossible toincorporate cooling surface (for instance in the form of cooling tubes)having an area that is sufficiently large for adequate removal of theheat, as a result of which the temperature in the reactor may rise toundesirably high values. For that reason, in such cases,sometimes use ismade of a tubular vessel containing an inner tube as reactor. The liquidpresent in a reactor of this type is circulated by pumping and withinthe inner tube moves in one direction and between the inner tube and thewall of the tubular vessel in the opposite direction. By installingcooling tubes parallel to the long axis of the reactor-which tubes maybe fitted both on the inside and on the outside of the inner tube-a verylarge cooling surface may be obtained so that the contents of thereactor can be prevented from rising to excessive temperature values. Ina reactor of this type, that is to say, where liquid is circulated, thesupply to the liquid of a sufficiently large quantity of gas that ispresent over the liquid presents a problem, particularly when thereactor is mounted vertically, because the area of the gas-liquidinterface is comparatively small and because it is impossible to imparta motion to that gas-liquid interface which is sufficiently vigorous toensure that the desired quantity of the gas that is present over theliquid is taken up in the liquid.

It is true that provision for sufficient gas to be taken up in theliquid may be made by introducing gas under pressure at one or moreplaces in the reactor where liquid is present, but this requirescomplicated equipment for accurate metering of the gas and, in addition,there is a risk of too much gas being introduced into the liquid andremaining therein when the consumption of the gas decreases (forinstance owing to a decrease in the rate of the gas-consuming reaction).When too much gas is present in the liquid, then, owing to the formationof large gas bubbles, the circulation of the liquid may be impeded and,as a result, excessive temperatures may occur locally.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod and apparatus by which gas is taken up in a liquid to asufficient extent and by which the quantity of gas that is present in aliquid is controlled automatically.

According to the invention, the gas is introduced through a tube belowthe the surface of the liquid, the quantity of gas in the circulatingstream of liquid being 0 controlled by adjusting the velocity of thestream of liquid and/or the depth to which the tube is immersed.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a vertical diagrammaticview of suitable apparatus for carrying out the method of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing,themethod according to our invention may be carried out in manydifferent types of apparatus, provided these types of an apparatus meetthe conditions that it should be possible for liquid to be circulated inthem and that it should be possible for a gas to be present over thecirculating liquid.

Such apparatus is represented diagrammatically in the drawing. Thisapparatus consists of a cylindrical vessel 1 that is mounted verticallyand is closed on either end. The vessel is provided with at least oneliquid supply device 2 and at least one liquid discharge device 3. Inaddition, at the upper end of the vessel there is present at least onegas supply device 4. Inside the vessel, in a coaxial position relativeto it, there is present an inner tube 5 which at or near its endscommunicates with the vessel. In the vessel 1, one or more pumps orstirrers 6 are provided by which the liquid may be made to circulatethrough the interior 7 of the inner tube 5 and the annular channel 8outside the inner tube 5. The apparatus is characterized by the presenceof at least one tube 9 in the vessel 1 near the upper end, each tube 9having a mouth 10 near the upper end of the vessel 1 opening into thegas that is present in the upper part of the vessel 1, and a mouth 11opening below the surface of the liquid that may be made to circulate.The liquid-gas interface is represented by 12; the direction of thearrows l3, l4 and 15 indicates the direction of flow of the liquidwithin vessel 1. Only one such tube 9 is illustrated in the drawing forconvenience of illustration.

To ensure an optimum uptake of the gas in the liquid, the mouth 11 ofthe tube 9 below the surface 12 of the liquid is so arranged that theoutward perpendicular on the plane of the mouth 11 of the tube 9 is atan angle of 0 (0 and 90 being included) to the direction of flow of theliquid. Mostly, use is made of a tube 9 of which the section before themouth below the surface of the liquid is horizontal or substantiallyhorizontal.

It is an advantage that downstream of the mouth 11 of the tube 9 belowthe surface 12 of the liquid, there are one or a plurality of baffles 16which facilitate the reversal of the direction of the flow of theliquid.

The inner tube 5 may partly rest on the lower end of the vessel 1 and/orbe connected to the upper end of the vessel 1, provided that thepossibility of circulation of liquid through the inner tube and theannular channel 8 outside that tube 5 is retained by arranging foropenings to remain in the inner tube 5.

Generally, in all supply and discharge devices, conventional valves 17,18, 19 and are provided as indicated, by which the rate of supply anddischarge of liquid or gas may be controlled.

The number of liquid supply devices 2 on the vessel 1 may be larger thanone and this is of importance particularly when one wants to supply morethan one liquid. Thus, in the preparation of carboxylic acids fromolefins, water and carbon monoxide under the influence of an acidcatalyst, the olefin and the catalyst may be supplied separately to thevessel, each by one or more supply devices The number of liquiddischarge devices 2 also may be larger than one, but this will not ingeneral be the case. The construction of the discharge device 3 ispreferably such that liquid is, but gas is not discharged. Provision forthis may be made in known ways, for instance by. means of an overflowprovided with a liquid seal (not shown).

Naturally, it is also possible for more than one gas supply device 4 tobe present.

In a number of cases (for example, in the continuous preparation ofcarboxylic acids as described hereinabove), it may be desired todischarge gas also from the apparatus, for which purpose a gas dischargedevice 21 may be provided.

Gas is taken up only if between the velocity of the stream of liquid andthe depth of immersion of the tube 9 a relationship exists that can beformulated as follows: V, a c-g-Ah. In this formula V, represents thevelocity of the stream of liquid at the location of the mouth 11 of thetube 9 below the surface 12 of the liquid, g the acceleration due togravity, A h the difference in level between the liquid-gas interface 12and the location of the mouth 11 of the tube 9 below the surface 12 ofthe liquid, and c a constant whose magnitude is mostly of the order oftwo if in expressing the quantities V,, g and A h the same units oflength and of time are used. By increasing V, and/or decreasing A h, thequantity of gas taken up in the liquid may be made to increase.

The quantity of gas that is present in the liquid in the form of bubblesmay be controlled by adjusting the velocity of the stream of liquidand/or the depth of immersion of the tube 9 in the liquid. Mostly thetube 9 is fitted in the vessel 1 so as to have a fixed position; thedepth of immersion may then be varied by filling the vessel 1 withliquid up to a higher or to a lower level. The velocity of the stream ofliquid may in the first instance be controlled by the speed of the pumpor the stirrer 6 to be used for effecting the circulation.

The velocity V, of the stream of liquid at the location of the mouth 11of the tube 9 below the surface 12 of the liquid is dependent not onlyon the speed of the pump or the stirrer 6 providing for the circulation,but also on the quantity of gas in the form of bubbles that is presentin the liquid; at a given speed of rotation, the value of V, is smalleraccording as the quantity of gas in the form of bubbles is larger.

The gas is drawn by the circulating stream of liquid from the tube 9 viathe mouth 11 below the surface 12 of the liquid, taken along in the formof bubbles by the circulating liquid, and, from this liquid, partlyconsumed, for instance, by a reaction taking place. After the liquid hascirculated once, little or no gas is present in the liquid that passesthe mouth 11 of the tube 9 below the surface 12 of the liquid. Since, asexplained above, V, is a function of the quantity of gas in the form ofbubbles that is present in the liquid, under conditions of constant gasconsumption, after some time a situation will arise where the said gasconsumption and the quantity of gas taken up in the liquid via the tube9 are in equilibrium, the equilibriumbeing such that the total quantityof gas in the liquid remains constant.

When the reaction rate decreases, the quantity of gas that is present inthe liquid will temporarily increase, but because this results in adecrease in V, and, hence, in less gas being taken up via the tube,after a short time a situation of equilibrium will arise again where acertain quantity of gas in the form of bubbles is present in the liquid.When the gas consumption increases, for instance by an increase in therate of the reaction, then the velocity of the liquid will increase, andmore gas will be taken up in the liquid via the tube 9; thus, here alsothere is automatic control of the stock of gas in the liquid.

It should be possible for gas to be taken up via the tube 9, that is tosay, the tube 9 is preferably mounted so that the stream of liquidexerts suction on the contents of the tube 9. With a view to promotingthe suction effect, it is advantageous for the tube 9 to be cut off insuch a way that the outward perpendicular on the plane of the mouth ofthe tube is at an angle of 0 (0 and 90 included) tothe direction of flowof the liquid.

Generally, provision may be made for a part of the tube 9 below thesurface 12 of the liquid to be in an entirely or substantiallyhorizontal position, the mouth 11 of the tube 9 in the liquid beinglocated in the horizontal section of the tube 9.

This method may be employed particularly with advantage in those caseswhere the direction of the circulating stream of liquid is substantiallyvertical, that is to say, where the distance in the vertical directioncovered by the stream of liquid when having circulated once is largerthan that in the horizontal direction.

Naturally, the invention is not restricted to the use of a single tube9: a plurality of tubes, for instance six or eight, may be used, oneextreme end-of these tubes being located below the surface 12 of theliquid, the other extreme end being located above the surface 12 in thegas that is present over the liquid.

Although the gas may be withdrawn from the bubbles without taking partin a reaction (for example, by physical binding) the method according tomy invention is used preferably in processes where a reaction takesplace in the circulating stream of liquid, in which reaction gas istaken up from the bubbles.

The method of my invention is particularly suited for the preparation ofcarboxylic acids or derivatives thereof with the aid of reactions inwhich carbon monoxide is bound. An attractive method for this purposeconsists in reacting one or more olefinically unsaturated compounds suchas olefins (or precursors thereof, such as alcohols and ethers) withwater (or an alcohol) and carbon monoxide in the presence of a stronglyacid component that serves as catalyst. In a preferred embodiment of themethod according to the invention where carboxylic acids are formed, thecirculating stream of liquid contains a strongly acid watercontainingcomponent to which in one or more locations an olefinically unsaturatedcompound is supplied, the gas consisting entirely or substantially ofcarbon monoxide.

Suitable strongly acid water-containing components are, for instance,mixtures of hydrogen fluoride and water, sulphuric acid and water, borontrifluoride and water, but preference is given to mixtures of borontrifluoride, phosphoric acid and water. Particularly suitable aremixtures of boron trifluoride, phosphoric acid and water in which themolar ratio between water and boron trifluoride lies in the rangebetween 1 l and 2.3 1, and the molar ratio between boron trifluoride andphosphoric acid in the range between 2 l and 20 l, but other ratios arealso well suitable and by no means excluded.

Very suitable olefinically unsaturated compounds are monolefins ormixtures thereof, particularly monoolefins with from two to 20 carbonatoms. Preference is given to monoolefins from which pivalic acid canform by reaction with CO and water, namely isobutene and oligomers ofisobutene, such as the dimer and the trimer.

Instead of, or in combination with, the monoolefins, precursors thereof,such as the alcohols which by separating off water might be caused tochange into the relevant olefin or ethers which might conceivably haveformed by separation of one molecule of water from two alcoholsmolecules, may also be used as starting material for the formation ofcarboxylic acids.

In the preparation of carboxylic acids as described hereinbeforetemperatures are applied that lie, for instance, in the range below 150C., preferably between 60 and 120 C.; very suitable pressures are thosebetween and 250 kg/cm, particularly those between 30 and 150 kg/cm".

The gas being present over the liquid may contain, in addition to carbonmonoxide, one or more inert gases (for instance nitrogen), but it ispreferred that this gas for the preparation of carboxylic acids in theway described hereinbefore should consist entirely or substantially ofcarbon monoxide. The olefinically unsaturated compounds added to thereactor contents may contain inert solvents such as hydrocarbons, forinstance pentane.

When the reaction has ended, the carboxylic acid formed may be obtainedby phase separation of the reactor contents into an organiccarboxylic-acid-containing phase and an inorganic phase containing theacid catalyst, which phase separation may be carried out in an apparatussuitable for this purpose, for instance a separating vessel or acentrifuge.

It is preferred that the preparation of the carboxylic acid be carriedout continuously, that is to say that the olefinically unsaturatedcompound is introduced continuously into the reactor and that reactorcontents are withdrawn continuously. In the withdrawn reactor contents,the organic carboxylic-acid-containing phase and the inorganic phasecontaining the acid catalyst are separated from each other and theinorganic phase is recycled to the reactor. As water from the inorganicphase has been used in the formation of the carboxylic acid, a quantityof water equal or substantially equal to the quantity of water consumedis supplied to the reactor, either together with the recycle inorganicphase or 'not.

It is very suitable for the phases to be separated to be discharged fromthe reactor with the aid of an overflow.

Owing to the consumption of carbon monoxide in the formation of thecarboxylic acid, the pressure in the reactor tends to drop; byreplenishment of carbon monoxide under pressure, preferably likewisecontinuously, the pressure may be kept at the desired level. In thecontinuous preparation of the carboxylic acid by the method according tomy invention (in which the reactor is continually replenished withcarbon monoxide under pressure) impurities present in the carbonmonoxide (for example, inert gases) will in the long run accumulate inthe gas cap over the liquid. For that reason it is advisable, thatduring the reaction small quantities of the gas in the gas cap over theliquid should be drawn off continuously or discontinuously.

The method according to my invention is also eminently suitable for thepreparation of esters from olefins (or precursors thereof). Esters areprepared in a manner analogous to that described hereinabove for thepreparation of carboxylic acids, but, instead of water, an alcohol isused, preferably a primary or secondary alcohol, for instance methanol,ethanol, propanol, isopropanol. As in the ester formation, tertiaryolefins (or precursors thereof) are incorporated in that part of theester which is derived from the acid more easily than primary orsecondary alcohols do, an ester can be prepared selectively from atertiary carboxylic acid and a primary or secondary alcohol. Thus, forinstance, with methanol as alcohol and isobutene or diisobutene asolefin, methyl pivalate is formed.

I claim as my invention:

1. In a process for producing carboxylic acids at a temperature below C.and at a pressure between 10 and 250 kg/cm by contacting carbon monoxidegas with a liquid mixture of a. monoolefin of from two to 20 carbonatoms; and

b. catalyst consisting essentially of water, boron trifluoride, andphosphoric acid, in which catalyst i. the water to boron trifluoridemolar ratio is from 1:1 to 2.3:1 and ii. the boron trifluoride tophosphoric acid molar ratio is from 2:1 to 20:1, the improvement of a.maintaining carbon monoxide gas above and in contact with the liquidmixture;

b. vertically circulating the liquid mixture; and

c. introducing carbon monoxide gas into the vertically circulatingliquid mixture at a depth below the gas-liquid mixture interface definedby wherein V is the velocity of the vertically circulating liquidmixture at the point of carbon monoxide gas introduction,

g is acceleration due to gravity,

Ah is the vertical distance below the gas-liquid mixture interface atwhich carbon monoxide gas is introduced into the vertically circulatingliquid mixture, and

c is a constant having a value of 2 where V g, and Ah are defined inconsistent units of length and time.

2. The process of claim 1 wherein the vertically circulating liquidmixture periodically passes a. upwardly within a tube substantiallycoaxially disposed within the reaction vessel and b. downwardly throughthe annulus between the tube and the walls of the reaction vessel.

3. The process of claim 2 wherein carbon monoxide gas is introduced intovertically circulating liquid mixture at a location within the coaxiallydisposed tube.

4. The process of claim 1 wherein carbon monoxide gas is introduced intovertically circulating liquid mixture at an angle of to 90, inclusive,relative to the direction of flow of vertically circulating liquidmixture.

5. The process of claim 4 wherein carbon monoxide gas is introduced intovertically circulating liquid mixture at an angle of 90 relative to thedirection of flow of vertically circulating liquid mixture.

6. The process of claim 1 wherein the monoolefin of from two to 20carbon atoms is selected from the group consisting of isobutene,isobutene dimer, isobutene trimer, and their mixtures.

7. In a process for producing esters of carboxylic acids at atemperature below 150 C. and at a pressure between and 250 kg/cm bycontacting carbon monoxide with a liquid mixture of a. monoolefin offrom two to carbon atoms; and

b. alcohol, boron trifluoride and phosphoric acid;

i. the alcohol being selected from the group consisting of methanol,ethanol, propane and isopropanol; 'ii. the alcohol to boron trifluoridemolar ratio being from 1:1 to 23:1; and iii. the boron trifluoride tophosphoric acid molar ratio being from 2:1 to 20:1, the improvement ofa. maintaining carbon monoxide gas above and in contact with the liquidmixture; b. vertically circulating the liquid mixture; and c.introducing carbon monoxide gas into the vertically circulating liquidmixture at a depth below the gas-liquid mixture interface defined by 5wherein V is the velocity of the vertically circulating liquid mixtureat the point of carbon monoxide introduc tion,

g is acceleration due to gravity,

Ah is the vertical distance below the gas-liquid mixture interface atwhich carbon monoxide gas is introduced into the vertically circulatingliquid mixture, and

c is a constant having a value of 2 where V g, and Ah are defined inconsistant units of length and time.

8. The process of claim 7 wherein the monoolefin of from two to 20carbon atoms is selected from the group consisting of isobutene,isobutene dimer, isobutene trimer, and their mixtures.

2. The process of claim 1 wherein the vertically circulating liquidmixture periodically passes a. upwardly within a tube substantiallycoaxially disposed within the reaction vessel and b. downwardly throughthe annulus between the tube and the walls of the reaction vessel. 3.The process of claim 2 wherein carbon monoxide gas is introduced intovertically circulating liquid mixture at a location within the coaxiallydisposed tube.
 4. The process of claim 1 wherein carbon monoxide gas isintroduced into vertically circulating liquid mixture at an angle of 0*to 90*, inclusive, relative to the direction of flow of verticallycirculating liquid mixture.
 5. The process of claim 4 wherein carbonmonoxide gas is introduced into vertically circulating liquid mixture atan angle of 90* relative to the direction of flow of verticallycirculating liquid mixture.
 6. The process of claim 1 wherein themonoolefin of from two to 20 carbon atoms is selected from the groupconsisting of isobutene, isobutene dimer, isobutene trimer, and theirmixtures.
 7. In a process for producing esters of carboxylic acids at atemperature below 150*C. and at a pressure between 10 and 250 kg/cm2 bycontacting carbon monoxide with a liquid mixture of a. monoolefin offrom two to 20 carbon atoms; and b. alcohol, boron trifluoride andphosphoric acid; i. the alcohol being selected from the group consistingof methanol, ethanol, propane and isopropanol; ii. the alcohol to borontrifluoride molar ratio being from 1:1 to 2.3:1; and iii. the borontrifluoride to phosphoric acid molar ratio being from 2:1 to 20:1, theimprovement of a. maintaining carbon monoxide gas above and in contactwith the liquid mixture; b. vertically circulating the liquid mixture;and c. introducing carbon monoxide gas into the vertically circulatingliquid mixture at a depth below the gas-liquid mixture interface definedby V12 > or = c . g . Delta h wherein V1 is the velocity of thevertically circulating liquid mixture at the point of carbon monoxideintroduction, g is acceleration due to gravity, Delta h is the verticaldistance below the gas-liquid mixture interface at which carbon monoxidegas is introduced into the vertically circulating liquid mixture, and cis a constant having a value of 2 where V1, g, and Delta h are definedin consistant units of length and time.
 8. The process of claim 7wherein the monoolefin of from two to 20 carbOn atoms is selected fromthe group consisting of isobutene, isobutene dimer, isobutene trimer,and their mixtures.